For decades, the effects of lunar cycles on human behaviors and biological processes, including sleep habits, mental illness, and menstrual cycles, have been hotly debated.^1,2 For many marine creatures, however, the biological significance of lunar rhythms is more firmly established: Certain corals, crabs, and pufferfish use the phases of the moon to coordinate the mass spawning events that are necessary to produce the next generation.³

The underlying biology that allows these animals to sync up with lunar rhythms remains largely mysterious. In chronobiologist Kristin Tessmar-Raible’s laboratory at the University of Vienna, the romantic activities of Platynereis dumerilii, a small orange seafaring bristle worm, help shed light on this phenomenon.

For organisms like the bristle worm that rely on external fertilization—in which the egg and sperm meet outside of the body—reproductive success is maximized when everyone releases their gametes into the water column at approximately the same time, said Tessmar-Raible. Thus, the worms need to “schedule” both a time and date for the event. For this particular species, spawning events are much more likely to take place a few days after the full moon.

Decades of research have demonstrated that circadian rhythms are dictated by internal molecular clocks that respond to external cues, such as light. Tessmar-Raible showed that the worms’ circalunar clocks functioned in a similar way: After two months of exposure to an artificial lunar cycle, the worms maintained the appropriate reproductive rhythms for several months in the absence of nocturnal light, indicating a faithful internal clock.⁴ On the other hand, worms that had never been given the correct lunar light cues failed to develop these reproductive rhythms. “They use the full moon light to synchronize their inner calendars,” said Tessmar-Raible.

In order to synchronize with lunar cycles, worms need a way to distinguish between the light of the full moon and the light of the sun. The researchers identified a worm photoreceptor protein called L-Cryptochrome, or L-Cry, that displays different biochemical responses—and localizes to different regions within the cell—based on the intensity and duration of the light.⁵ Tessmar-Raible and her colleagues hypothesize that L-Cry functions as a kind of gatekeeper, determining which kind of light most efficiently resets the worms’ internal clocks.

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